Silver alloy foil for interconnector for solar cell
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention pertains to a silver alloy foil for use in an interconnector of a Si or Ga-As solar cell employed in satellites or the like, and in particular to a foil which is less susceptible to softening even when the solar cell is exposed to temperature cycling. 
     2. Prior Art 
     As disclosed in Japanese Patent Application 18-Month Publication No. 62-101441, the same applicant developed a foil for an interconnector of a solar cell which was made of silver alloy containing 1O to 1OOO ppm by weight (hereinafter referred to as &#34;ppm&#34;) of calcium, balance silver and unavoidable impurities. In this silver alloy foil, the characteristics do not change with the passage of time as long as the circumstances remain unchanged. 
     However, when the above silver alloy foil is exposed to severe temperature cycling which may occur in outer space, the characteristics, in particular the hardness, gradually decrease as time passes, and hence it is difficult to maintain the characteristics so as not to deteriorate over a prolonged period of time. 
     SUMMARY OF THE INVENTION 
     It is therefore an object of the present invention to provide a silver alloy foil for use in an interconnector of a solar cell which is less susceptible to softening even when the solar cell is exposed to temperature cycling. 
     According to the present invention, there is provided a foil for an interconnector of a solar cell made of a silver alloy which contains 1O ppm to 750 ppm of at least one element selected from the group consisting of beryllium (Be), lanthanum (La) and indium (In); balance silver (Ag) and unavoidable impurities. 
     The above silver alloy foil is less susceptible to softening even when the solar cell is exposed to temperature cycling. However, when 1O ppm to 1,OOO ppm of calcium is further added, the hardness does not decrease even with the passage of time, to thereby exhibit superior characteristics for a prolonged period of time. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     After an extensive study on the silver alloy foil for use in an interconnector of a solar cell, the inventors have come to know that if beryllium, lanthanum and indium are added as constituents elements, the resulting alloy foil becomes less susceptible to softening even when the solar cell is exposed to temperature cycling. 
     A silver alloy foil in accordance with the present invention is developed in view of the foregoing, and is characterized in that it is made of a silver alloy which contains 10 ppm to 750 ppm of at least one element selected from the group consisting of beryllium, lanthanum and indium; balance silver and unavoidable impurities. 
     Beryllium, lanthanum and indium are effective to prevent the decrease of the hardness when the solar cell is exposed to temperature cycling. However, if the content of at least one of beryllium, lanthanum and indium is less than 1O ppm, the reduction in the hardness due to temperature cycling cannot be sufficiently prevented. In contrast, if the content exceeds 750 ppm, the conductivity deteriorates abruptly. In addition, since the oxdized film tends to be easily formed, it is very difficult to carry out spot welding. Furthermore, it becomes difficult to work the foil due to its unduly increased hardness. 
     The silver alloy foil in accordance with the present invention exhibits a superior conductivity, and is less susceptible to softening even when exposed to severe temperature cycling, so that the characteristics obtained immediately after its manufacture can be maintained for a prolonged period of time. Therefore, the alloy foil can exhibit a superior performance for a prolonged period of time and can be successfully utilized for an interconnector of a solar cell. 
     In the foregoing, calcium may be preferably added since calcium serves to prevent the alloy from being softened with the passage of time. However, if the calcium content is less than 10 ppm, such a effect cannot be obtained. On the other hand, if the calcium content exceeds 1,OOO ppm, its conductivity decreases abruptly. Therefore, it is preferable that the content ranges from 10 ppm to 1,000 ppm. In addition, when calcium is also present, the function of the elements such as beryllium, lanthanum and indium is enhanced even further, so that the superior characteristics of the alloy can be maintained for a further prolonged period of time. 
     The present invention will now be described in more detail by way of examples. 
     EXAMPLE 1 
     A usual vacuum melting furnace was used to prepare silver alloy melts having various compositions as set forth in TABLES 1-1 and 1-2. The melts were cast into ingots each having a thickness of 1O mm, a width of 120 mm and a length of 200 mm, and the opposite surfaces of each ingot were machined, so that ingots of 8 mm in thickness were obtained. Then, for each ingot, Micro Vickers Hardness was measured under a load of 5 g, and conductivity was measured using an eddy current conductometer. Thereafter, each ingot was subjected to cold rolling under ordinary conditions to produce silver alloy foils 1 to 25 in accordance with the present invention and prior art silver alloy foils 1 to 5 each having a thickness of 0.03 mm. 
     Subsequently, immediately after the manufacture, the Micro Vickers Hardness was measured under a load of 5 g for each foil. In addition, each foil was held at 3OO° C. for 30 minutes, and was immediately immersed and cooled in water at room temperature. These changes were repeated one hundred times, and then the Micro Vickers Hardness was measured under a load of 5 g. The results are set forth in TABLES 1-1 and 1-2. 
     As will be seen from TABLES 1-1 and 1-2, in the silver alloy foils 1 to 25 of the invention, the decrease in the hardness of the foil subjected to the temperature cycling, which was more severe than those encountered in the space, is extremely small as compared with the hardness of the alloy immediately after the manufacture, which was obtained due to hardening during cold drawing. In addition, the alloy foils 1 to 25 of the invention have great conductivity of 102% or more. In contrast, all of the prior art alloy foils 1 to 5 are less resistant to the temperature cycling, resulting in a considerable decrease in hardness. 
     EXAMPLE 2 
     A usual vacuum melting furnace was used to prepare silver alloy melts having compositions as set forth in TABLES 2-1 and 2-2. Then, the same procedures as in EXAMPLE 1 were repeated to produce silver alloy foils 26 to 50 in accordance with the present invention and prior art silver alloy foils 6 to 10 each having a thickness of 0.03 mm. 
     Subsequently, as was the case with EXAMPLE 1, the testing of hardness was carried out. Furthermore, the hardness of each alloy foil was measured under a load of 5 g when the alloy foil manufactured was held at the room temperature for six months. The results are also set forth in TABLES 2-1 and 2-2. 
     As will be seen from TABLES 2-1 and 2-2, in the silver alloy foils 26 to 50 of the invention, the decrease in the hardness of the foil subjected to the temperature cycling is extremely small as compared with the hardness of the alloy immediately after the manufacture, and the decrease in the hardness after six months passed is also very small. In addition, the alloy foils of the invention have great conductivity of 94% or more. On the other hand, although the prior art alloy foils 6 to 10 are less susceptible to softening and have great conductivity at room temperature, they are subject to considerable decrease in hardness when exposed to severe temperature cycling. 
     
                                           TABLE 1                                 
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Composition of Ag Alloy (ppm)                                             
                    Ingot       Hardness of Foil (Hv)                     
              Ag +  Hardness                                              
                         Conductivity                                     
                                Immediately                               
                                          Subjected to                    
Be   La In Ca Impurities                                                  
                    (Hv) (IACS %)                                         
                                After Manufacture                         
                                          Temperature Cycling             
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Ag Foil of the Invention                                                  
 1                                                                        
   10.8                                                                   
     -- -- -- other 40   117    92        83                              
 2                                                                        
  185.2                                                                   
     -- -- -- other 48   110    99        95                              
 3                                                                        
  396.4                                                                   
     -- -- -- other 48   109    100       94                              
 4                                                                        
  596.0                                                                   
     -- -- -- other 52   107    101       94                              
 5                                                                        
  744.8                                                                   
     -- -- -- other 58   102    104       95                              
 6                                                                        
  --  30.4                                                                
        -- -- other 42   116    91        83                              
 7                                                                        
  -- 232.1                                                                
        -- -- other 45   112    94        87                              
 8                                                                        
  -- 374.6                                                                
        -- -- other 47   110    94        89                              
 9                                                                        
  -- 553.2                                                                
        -- -- other 47   111    96        90                              
10                                                                        
  -- 725.6                                                                
        -- -- other 52   107    96        90                              
11                                                                        
  -- --  13.2                                                             
           -- other 40   112    92        88                              
12                                                                        
  -- -- 140.6                                                             
           -- other 42   112    94        88                              
13                                                                        
  -- -- 281.3                                                             
           -- other 44   111    95        90                              
14                                                                        
  -- -- 570.6                                                             
           -- other 50   109    94        89                              
15                                                                        
  -- -- 711.7                                                             
           -- other 57   107    97        90                              
16                                                                        
   50.4                                                                   
      36.8                                                                
        -- -- other 44   108    92        90                              
17                                                                        
  110.2                                                                   
     -- 101.4                                                             
           -- other 51   107    100       94                              
18                                                                        
  -- 151.5                                                                
        153.3                                                             
           -- other 52   110    97        92                              
19                                                                        
  234.1                                                                   
     229.5                                                                
        -- -- other 54   109    101       97                              
20                                                                        
  324.5                                                                   
     -- 312.1                                                             
           -- other 57   107    106       101                             
21                                                                        
   4.2                                                                    
      4.4                                                                 
         9.8                                                              
           -- other 48   107    98        92                              
22                                                                        
   60.8                                                                   
      54.2                                                                
         70.5                                                             
           -- other 52   108    101       97                              
23                                                                        
  130.2                                                                   
     154.1                                                                
        126.7                                                             
           -- other 51   110    103       98                              
24                                                                        
  201.4                                                                   
     205.4                                                                
        211.8                                                             
           -- other 53   107    108       101                             
25                                                                        
  233.1                                                                   
     241.5                                                                
        254.6                                                             
           -- other 50   106    108       101                             
Prior Art Ag Foil                                                         
 1                                                                        
  -- -- --  10.7                                                          
              other 39   115    98        36                              
 2                                                                        
  -- -- -- 196.3                                                          
              other 46   110    101       42                              
 3                                                                        
  -- -- -- 396.2                                                          
              other 53   106    106       61                              
 4                                                                        
  -- -- -- 601.7                                                          
              other 64   100    108       63                              
 5                                                                        
  -- -- -- 741.6                                                          
              other 71    99    110       67                              
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                                           TABLE 2                                 
__________________________________________________________________________
Composition of Ag Alloy (ppm)                                             
                    Ingot       Hardness of Foil (Hv)                     
              Ag +  Hardness                                              
                         Conductivity                                     
                                Immediately                               
                                          6 Months  Subjected to          
Ca   In Be La Impurities                                                  
                    (Hv) (IACS %)                                         
                                After Manufacture                         
                                          After Manufacture               
                                                    Temperature           
__________________________________________________________________________
                                                    Cycling               
Ag Foil of the Invention                                                  
26                                                                        
   10.6                                                                   
     10.6                                                                 
        -- -- other 40   115     98        98        91                   
27                                                                        
   12.3                                                                   
     -- 746.5                                                             
           -- other 44   112     98        97        92                   
28                                                                        
   11.6                                                                   
     --  96.3                                                             
            99.2                                                          
              other 43   114     97        94        90                   
29                                                                        
   13.2                                                                   
     192.6                                                                
        -- 213.6                                                          
              other 47   115     99        97        92                   
30                                                                        
   15.8                                                                   
     208.3                                                                
        204.3                                                             
           186.4                                                          
              other 50   110    101       100        93                   
31                                                                        
  251.3                                                                   
     --  30.9                                                             
           -- other 51   109    100        98        91                   
32                                                                        
  254.2                                                                   
     -- -- 593.1                                                          
              other 50   112    101        98        92                   
33                                                                        
  246.6                                                                   
      5.8                                                                 
         6.2                                                              
           -- other 48   107    106        99        95                   
34                                                                        
  250.3                                                                   
     745.1                                                                
        -- -- other 47   106    110       108       104                   
35                                                                        
  248.5                                                                   
      96.4                                                                
        102.3                                                             
           114.5                                                          
              other 51   102    109       107       105                   
36                                                                        
  496.2                                                                   
     -- --  54.1                                                          
              other 50   109    104       104       101                   
37                                                                        
  503.1                                                                   
     --  42.3                                                             
           -- other 49   107    108       103       100                   
38                                                                        
  514.6                                                                   
     324.1                                                                
        -- 302.1                                                          
              other 48   106    109       109       102                   
39                                                                        
  493.9                                                                   
     -- 304.1                                                             
           314.2                                                          
              other 49   109    108       105       100                   
40                                                                        
  499.0                                                                   
      6.8                                                                 
         10.9                                                             
           121.3                                                          
              other 52   101    112       110       102                   
41                                                                        
  750.4                                                                   
      68.4                                                                
        -- -- other 51   109    111       110       103                   
42                                                                        
  733.9                                                                   
     -- 213.1                                                             
           -- other 53   107    110       107       106                   
43                                                                        
  752.4                                                                   
     -- --  13.2                                                          
              other 53   107    107       102       100                   
44                                                                        
  760.1                                                                   
     304.6                                                                
        313.2                                                             
           -- other 60   109    116       109       107                   
45                                                                        
  752.6                                                                   
      60.8                                                                
         79.6                                                             
            96.4                                                          
              other 59   107    112       110       109                   
46                                                                        
  991.4                                                                   
     734.6                                                                
        -- -- other 58   104    110       109       107                   
47                                                                        
  992.1                                                                   
     -- --  99.4                                                          
              other 52   100    109       108       104                   
48                                                                        
  984.5                                                                   
     --  30.8                                                             
           194.1                                                          
              other 54    98    107       106       102                   
49                                                                        
  996.8                                                                   
     250.4                                                                
        -- 214.8                                                          
              other 56    94    110       107       101                   
50                                                                        
  964.6                                                                   
     413.2                                                                
         96.4                                                             
           124.6                                                          
              other 61    97    114       114       109                   
Prior Art Ag Foil                                                         
 6                                                                        
   11.2                                                                   
     -- -- -- other 39   115     98        90        49                   
 7                                                                        
  251.3                                                                   
     -- -- -- other 47   107     99        97        50                   
 8                                                                        
  506.5                                                                   
     -- -- -- other 56   105    113       111        48                   
 9                                                                        
  748.1                                                                   
     -- -- -- other 71   103    113       112        54                   
10                                                                        
  993.3                                                                   
     -- -- -- other 73   102    119       117        54                   
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